In the U.S. Pat. No. 3,981,481 granted Sept. 21, 1976, assigned to the same assignee as the present application, a bottom operable tank car valve is disclosed including a valve body attached to an opening in the bottom of the tank as shown in FIG. 1 of the drawings. The valve body includes an opening centrally thereof for loading and unloading the lading. A depending housing is attached to the valve body with mechanical fasteners which constitute a shear plane. The housing includes a loading and unloading spout extending radially outwardly from the outlet. An operator assembly for loading and unloading the tank is mounted within a bore in the lower portion of the housing. The operating assembly includes a lower operator which extends below the valve housing and which is adapted to be engaged by a suitable unloading tool. The lower operator has a polygon opening engaging with clearance a cooperating polygon head on an upper valve operator which extends upwardly within the opening in the valve body. A retainer assembly attached to the valve body maintains the upper operator vertically fixed and spaced from the walls of the valve body. The upper operator engages a depending member from a valve closure. The closure is movable between a closed position engaging the top surface of the retainer assembly, and an open position to load or unload the lading.
In accordance with one embodiment in the U.S. Pat. No. 3,981,481 the upper operator engaging the depending valve closure operator is located within the valve body at a point above the shear plane defined by the mechanical fasteners holding the housing in engagement with the valve body as shown in FIG. 10 and 11 of the U.S. Pat. No. 3,981,481 and FIG. 1 herein. Thus in the event of impact to the depending housing, the housing will shear off along the shear plane provided in the mechanical fasteners, and the lower operator will drop off, leaving in place the upper operator within the valve body. After such an impact, the car can be unloaded by attaching another housing or a plate to support the retainer and applying a suitable tool to the connecting portion on the upper operator to open the valve closure and unload the car.